Forming an electrical contact on an electronic component

ABSTRACT

A method of constructing an electrical contact on an electronic component comprises first forming a protruding electrically conducting stud at a contact location by wire bonding a metal wire to a contact pad of the component. The stud is then contacted with solder, without using a mask, so that a solder bump is deposited on and adheres to the metal stud to form a composite solder contact which is able to form with a contact of another component a solder joint which has good electrical and mechanical properties and which may be reliable fabricated at high density by a low cost method. An electronic component provided with such solder contacts and an electronics component package including such a component are also disclosed.

RELATED APPLICATIONS

[0001] This application claims priority to Singapore Application serialNo. 200100049-6, filed Jan. 4, 2001, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates to a method of forming an electricalcontact on an electronic component, such as the semiconductor chip orsubstrate of an electronic component package, particularly forapplication in flip-chip technology.

BACKGROUND OF THE INVENTION

[0003] With the increasing density of devices on a semiconductor chipand ever more complex applications there is a need to form ever smallerand more precise electrical connections when packaging suchsemiconductor chips, especially medium and higher end products with highinput/output counts.

[0004] In one conventional method, electrical connections in packagedelectronic components are made using electrical contacts in the form ofsolder bumps which are deposited by a solder deposition process oncontact pads of one of two components to be interconnected, the solderdeposition process requiring special alignment techniques, such as theapplication of a masking resist to the surface of the component todefine the individual locations at which solder bumps are to bedeposited. The masking layer then has to be subsequently removed afterthe solder has been deposited on the surface.

[0005] In another known method, solder paste is screen printed on thecontact pads of a component using a metal mask. The use of such metalmasks means that accurate alignment techniques are required to depositthe paste on individual pads and that the chance of bridging betweenpads increases as the pitch decreases.

[0006] In yet another known process for making electrical connections, acontact on one component is connected to a metal stud on anothercomponents using an electrically conducting adhesive material whichcontains metal particles. Because only a few of the metal particles inthe adhesive material make an electrical connection with the stud, theelectrical resistance of such an connection is rather high.

[0007] With the ever smaller size, higher packing density and complexityof semiconductor devices in semiconductor chips, the electroniccomponent packaging industry is increasingly being faced with the needto provide more closely packed electrical connections of smaller andsmaller area whilst maintaining the electrical isolation and integrityof the individual connections.

[0008] Because the required dimensions of solder joints are thereforesteadily shrinking and solders have an intrinsically low melting point,the electrical resistance, the mechanical strength and thesusceptibility of such joints to electro-migration are becomingincreasingly important factors.

[0009] It is becoming increasingly troublesome to form satisfactoryconnections of the required small size and at the required small pitchusing these known connection-forming techniques. For example, it isextremely difficult and relatively expensive to form solder bumps byconventional methods with a bump size of 75 μm or less at a pitch of 125μm or less.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide a method offorming an electrical contact on an electronic component which enablesstrong, low resistance connections of high integrity to be reliablyconstructed with a size of 100 μm or less and a pitch of 150 μm or less.

[0011] Accordingly, in one aspect, the invention provides a method ofconstructing an electrical contact on an electronic component,comprising providing a protruding electrically conducting core on thecomponent at a site where an electrical connection is to be made, andplacing the core in contact with molten solder, without using a mask, toform on the stud a solder bump which adheres to the stud.

[0012] In another aspect, the invention provides a method ofelectrically interconnecting two electronic components, comprisingforming an electrical contact on a contact pad of one of the componentsusing the method of the invention and forming a bond between theelectrical contact thus formed and a contact pad of the other component.

[0013] In a further aspect, the invention provides an electroniccomponent having an electrical contact which is formed by a protrudingelectrically conducting core and a solder bump formed on and adhering tothe core.

[0014] In yet another aspect, the invention provides an electroniccomponents package comprising a semiconductor chip and a mountingsubstrate, in which one of the chip and the substrate is a componenthaving electrical contacts formed in accordance with the invention, andthe other of the chip and substrate has contact pads bonded to theelectrical contacts of the one component by the solder bumps of thecontacts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order that the invention may be more readily understood, anembodiment thereof will now be described, by way of example, withreference to the accompanying illustrations, in which:

[0016]FIG. 1 is a plan view of an electronic component embodying thepresent invention in the form of a semiconductor test chip havingelectrical contacts formed by a method embodying the invention;

[0017]FIG. 2 is a plan view of the surface of the electronic componenton an enlarged scale, showing the fine pitch which can be achieved bythe method embodying the invention between adjacent electrical contactsin a row of such contacts;

[0018]FIG. 3 is on a still larger scale and shows the integrity and goodelectrical isolation which can be achieved between adjacent electricalcontacts formed in accordance with the method embodying the invention;

[0019]FIG. 4 is a view from above of a row of electrical contacts formedby the method embodying the present invention;

[0020]FIG. 5 is a schematic representation of a solder bond formedbetween a substrate and a semiconductor chip using an electrical contactprovided on the chip by the method embodying the present invention;

[0021]FIG. 6 is a cross-section taken through an electrical contactformed by the method embodying the invention; and

[0022]FIGS. 7 and 8 illustrate the results of shear testing anelectrical contact of an electronic component provided with such contactby the method embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] In one embodiment of the method of the present invention,protruding electrically conducting cores in the form of solid metalstuds 1 (see FIGS. 4, 5 and 6) are fabricated on contact pads 2 of asemiconductor chip 3 which is to be electrically interconnected with amounting substrate, which may be an organic substrate, in themanufacture of a packaged electronics product. The contact pads 2 arecommonly made of aluminium or copper and may possibly be coated withother materials. The studs 1 are formed from gold wire of 25.4 μmdiameter secured to the contact pads by means of a conventional wirebonding technique. The studs may be coined after they have been formedon the chip or may be left plain. Although the stud configurations shownin the drawings are linear arrays, both linear and two-dimensional studarrays may be fabricated.

[0024] The projecting gold studs 1 thus formed on the contact pads 2 arethen brought into contact with molten solder, for example by dipping ina solder bath, whereupon it is found that the solder selectively wetsand adheres only to the studs and not to the other areas of thesemiconductor chip 3. This selective wetting of the metal studs by thesolder effectively self-alignes the solder deposits without the need forany additional alignment steps, such as the application and developmentof photo-resists or application of some other kind of mask, e.g. a metalmask.

[0025] FIGS. 1 to 5 show the electrical contacts 5 which are formedafter dipping in molten solder and which comprise solder bumps 4deposited on and adhering to the gold studs 1 fabricated on thealuminium pads 2 of the semiconductor test chip 3. As will be seen fromthe Figures, the solder only wets and adheres to the gold studs 1,leaving the aluminium pads 2 and the passivation surface 6 of thesemiconductor chip 3 free of solder.

[0026]FIGS. 2 and 3 show that electrical contacts 5 formed in accordancewith a method embodying the invention may be fabricated in closeproximity to one another whilst still maintaining their structural andelectrical integrity. In particular, FIGS. 2 and 3 show that contacts 5having solder bumps 4 with a dimension of 75 μm can be fabricatedwithout any solder bridging between the solder bumps, this bumpdimension being comparable to or better than those achievable by theknown processes currently used by the electronic component packagingindustry.

[0027]FIG. 4 shows that solder can be selectively deposited with goodaccuracy on the top part of the gold stud 1.

[0028] After forming contacts 5 on a test chip 3 as described above, thechip was assembled with a substrate 7 (see FIG. 5). To this end,input/output pads 8 provided on the substrate 7 are masked with a soldermask 9 and joined to the solder bumps 4 of the electrical contacts 5 ofthe chip 3 to form solder joints which electrically interconnect thepads 2 and 8 of the chip 3 and substrate 7 as desired. In the resultingassembly, the solder bumps 4 of the chip contacts 5 formed a robustmetallic bond with the input/output pads 8 of the substrate 7.

[0029] To test the reliability of the interconnections, the electricalcontacts 5 were subjected to a microscopy study and shear tests wereconducted on the electrical contacts using an ASTM (American Society forTesting Materials) standard method.

[0030]FIG. 5 shows a schematic cross-section through the electricalcontacts 5 formed on the chip 3 by the studs 1 with the solder bumps 4adhering thereto. The shear tests revealed that the contacts had shearstrengths well above the minimum acceptable value. Moreover, asillustrated in FIGS. 6 and 7, the shear tests applied to the contacts 5resulted in fractures 6 occurring at the interface between the chip pads2 and the silicon of the semiconductor chip 3, suggesting the existenceof a very robust interface between the electrical contacts 5 and thepads 2 as well as strength within the contacts 5 themselves.

[0031] Whilst in the above described embodiment the electrical contactsare formed on a semiconductor chip they could, of course equally beformed on the substrate or on some other electronic component with whicha solder joint is to be formed.

[0032] Although the studs in the described embodiment are made of gold,this is but one example of an electrically conducting material which canbe used to form the solid core of a solder contact embodying theinvention. In particular, wires of copper, silver, platinum, palladiumor nickel or their alloys may be used to construct the electricallyconducting studs on which the solder bumps are deposited to form theelectrical contacts. Furthermore, it is envisaged that wires made of anyother suitable material coated with gold, copper, silver, platinum,palladium or nickel or their alloys could be used to construct theelectrically conducting studs. Moreover, it is envisaged that aplurality of studs may be stacked by wire bonding to increase the heightof the resulting solder joint.

[0033] The formation of the solder bumps on the electrically conductingstuds may be performed using any suitable process, such as dipping in asolder bath as described above or, for example, by wave soldering. Ifdesired or necessary in any particular case, the studs may also becontacted with the solder a plurality of times in order to build thesolder bump up to a required size.

[0034] It will be appreciated that electrical contacts embodying thepresent invention can be fabricated at relatively low cost using onlyequipment which is already standard in the electronic componentpackaging industry. In addition, the solder bumps are deposited on thestuds by a self-aligning process which obviates the need to use masks todefine the locations of the solder deposits, so that the method offabricating the contacts is in fact simpler in this respect thanexisting methods.

[0035] The use of contacts embodying the invention enables the formationof high strength bonds during assembly, even with electrical contactshaving a dimension of 75 μm or less, due to the combined effects of themechanical support provided by the solid stud itself and a measure ofsolute alloying of the stud material which occurs when the molten solderis deposited on the stud. The solute alloying of the stud material intothe solder of the solder bump also enhances the resistance of thecontact to electromigration, a potentially serious problem as the sizeof solder joints decreases.

[0036] The high strength and the integrity of contacts embodying theinvention significantly reduce the chances of the contacts collapsingduring assembly, one of the main factors in reducing yields in theelectronic component packaging industry. Moreover, the method by whichsuch contacts are formed means that solder bumps having dimensions of 75μm or less can be reliably produced at a pitch of 150 μm or less,thereby allowing the input/output counts of packaged semiconductor chipsto be increased.

[0037] It is envisaged that, where the component on which the electricalcontacts are to be formed is a chip, it may be advantageous to form thestuds on chip before a semiconductor wafer incorporating the chip isdiced to separate the individual chips contained in the wafer.

[0038] In the present specification “comprise” means “includes orconsists of” and “comprising” means “including or consisting of”.

[0039] The features disclosed in the foregoing description, or thefollowing claims, or the accompanying drawings, expressed in theirspecific forms or in terms of a means for performing the disclosedfunction, or a method or process for attaining the disclosed result, asappropriate, may, separately, or in any combination of such features, beutilised for realising the invention in diverse forms thereof.

What is claimed is:
 1. A method of constructing an electrical contact onan electronic component, comprising providing a protruding electricallyconducting core on the component at a site where an interconnection isto be made, and placing the core in contact with molten solder, withoutusing a mask, to form on the core a solder bump which adheres to thecore.
 2. A method according to claim 1, comprising forming theelectrically conducting core from a material selected from the groupconsisting of gold, copper, silver, platinum, palladium and nickel andtheir alloys.
 3. A method according to claim 1, comprising forming theelectrically conducting core by attaching a metal stud to the componentat the site where the interconnection is to be made.
 4. A methodaccording to claim 3, comprising attaching the metal stud to a contactpad of the component by wire bonding.
 5. A method according to claim 4,comprising forming the electrically conducting core from a materialselected from the group consisting of gold, copper, silver, platinum,palladium and nickel and their alloys.
 6. A method according to claim 3,comprising coining the metal stud after it has been formed on thecomponent.
 7. A method according to claim 6, comprising forming theelectrically conducting core from a material selected from the groupconsisting of gold, copper, silver, platinum, palladium and nickel andtheir alloys.
 8. A method according to claim 6, comprising forming themetal stud from a length of wire which is bonded to a contact pad of thecomponent and which is coated with a material selected from the groupconsisting of gold, copper, silver, platinum, palladium and nickel andtheir alloys.
 9. A method according to claim 3, comprising coining themetal stud after it has been formed on the component.
 10. A methodaccording to claim 9, comprising forming the electrically conductingcore from a material selected from the group consisting of gold, copper,silver, platinum, palladium and nickel and their alloys.
 11. A methodaccording to claim 9, comprising forming the metal stud from a length ofwire which is bonded to a contact pad of the component and which iscoated with a material selected from the group consisting of gold,copper, silver, platinum, palladium and nickel and their alloys.
 12. Amethod according to claim 3, comprising forming the electricallyconducting core from a material selected from the group consisting ofgold, copper, silver, platinum, palladium and nickel and their alloys.13. A method according to claim 3, comprising forming the metal studfrom a length of wire which is bonded to a contact pad of the componentand which is coated with a material selected from the group consistingof gold, copper, silver, platinum, palladium and nickel and theiralloys.
 14. A method according to any preceding claim, comprisingforming the solder bump by dipping the electrically conducting core intoa bath of molten solder.
 15. A method according to any one of claims 1to 13, comprising forming the solder bump by wave soldering.
 16. Amethod of interconnecting two electronic components, comprising formingan electrical contact by the method of any preceding claim on a contactpad of one of the components and forming a bond between the contact thusformed and a contact pad of the other component.
 17. An electroniccomponent having a contact which is formed by a protruding electricallyconducting core and a solder bump formed on and adhering to the core.18. An electronic component according to claim 17, in which the core ofthe contact is a metal stud.
 19. An electronic component according toclaim 18, in which the material of the metal stud is selected from thegroup consisting of gold, copper, silver, platinum, palladium and nickeland their alloys.
 20. An electronic component according to claim 18, inwhich the metal stud is formed by a length of wire bonded to a contactpad of the component.
 21. An electronic component according to claim 20,in which the material of the metal stud is selected from the groupconsisting of gold, copper, silver, platinum, palladium and nickel andtheir alloys.
 22. An electronic component according to claim 20, inwhich the wire is coated with a material selected from the groupconsisting of gold, copper, silver, platinum, palladium and nickel andtheir alloys.
 23. An electronic component according to any one of claims17 to 22, which component is a semiconductor chip.
 24. An electroniccomponent according to any one of claims 17 to 22, which component is asubstrate.
 25. A electronic components package comprising asemiconductor chip and a mounting substrate, in which one of the chipand the substrate has electrical contacts formed in accordance with anyone of claims 17 to 22, and the other of the chip and substrate hascontact pads bonded to the contacts by the solder bumps of the contacts.